Fuels containing vegetable oil derivatives have a reduced emission level and an increased biodegradability, but they habitually demonstrate a very high oxidation sensitivity and show very poor properties under cold conditions. Despite these drawbacks, sales of fuels containing these derivatives have increased significantly in recent years as the production of these derivatives becomes more competitive with conventional fuels derived from crude oil. In particular, the production of such fuels permits a more positive view of the future as oil reserves diminish. However, the poor properties of these components under cold conditions and their high oxidation sensitivity have for a long time constituted factors limiting their use in high concentrations in diesel fuels, as there is a risk that their physical and chemical characteristics will jeopardize the satisfactory operation of commercial engines. Taking account of these engines, it is at present difficult to use these derivatives at levels exceeding 5% in diesel fuels and obtain optimum efficiency.
These vegetable oil derivatives are often products of the transesterification of vegetable or animal oil triglycerides. They have a high cetane number corresponding to a good performance of the fuel upon combustion. For these derivatives, a cetane number of at least 47 is required according to United States standards (ASTM D 6751) and 51 in certain European countries (such as Germany for example). Since cetane numbers correlate with low nitrogen oxide emissions, the addition of these products can be very significant for the production of diesel fuels to which products obtained from vegetable and animal oils have been added.
G. Knothe et al., Fuel 82, 971-975 (2003) gives the cetane numbers of esters of many fatty acids such as palmitic acid, stearic acid, oleic or linoleic acid and in particular the beneficial effect of increasing the length and the saturated character of the chain on the cetane number, as is the case for esters of palmitic acid and stearic acid. It is observed moreover, that the linear or branched character of the ester has no effect.
In European application EP 1484385, a biofuel is described containing 100% by weight of a mixture of esters of palm oil, this mixture having a low pour point, particularly suitable for cold countries without addition of additives. This biofuel is produced by esterification of a mixture of C18 (stearic), C18:1 (oleic) and C18:2 (linoleic) fatty acids by methanol or ethanol, followed by the fractional distillation of the methyl and ethyl esters, finally crystallization. The biofuel is prepared by mixing these fractions originating from the distillation of palm oil. It is used as a fuel, on environmental principles, as a substitute for diesel fuels, obtained from palm oil and standard mixtures of palm oil esters.
Olefinic fatty acid esters contain more than 16 carbon atoms and in particular methyl esters are known for their good properties under cold conditions but also for their high oxidation sensitivity. Moreover, although they have the advantage of high cetane numbers, saturated fatty acid esters with up to 18 carbon atoms are known for their tendency to deteriorate the flow properties of biodiesels.